There are a couple of different hypotheses concerning a background medium of that everything would propagate through. I’m only giving a very crude description below.
Some scientists believe that the true background medium isn’t the old version of the aether, but is really made up of sub-quantum particles, and is the ultimate level of reality. Some scientists believe that what’s termed as the alleged dark energy having long range repulsion powers that’s pushing the universe farther apart is really sub-quantum particles/primaries made up of positive and negative mass. Their collisions would produce a unique energy gain due to their characteristics.
General Relativity does not acknowledge a background medium would exist, though according to some scientists it could exist, if its motion was relative itself. Technically the ‘void’ of outer space is filled with virtual particles, so perhaps this too could be considered to be a background medium of sorts.
Some scientists have proposed some experiments utilizing some electrical components to try to discredit relativity. Some of these experiments involved induction on coils, and even oscillating capacitors. The entire point of the aether theory was that it was based on absolute motion over relative motion, and some scientists had critiqued the Michelson-Morley experiment.